1. Field of the Invention
The present invention relates to a cardlike optical recording medium.
2. Related Background Art
In recent years, as society is progressively informationalized, various information recording carriers and optical information recording-reproducing apparatus for optically recording and reproducing information, including optical discs, optical cards, optical tapes, etc., have been proposed as a means for dealing with a great variety of information with high efficiency. The above information recording carriers include those which can detect the binary-coded information and convert it to the change of strength of reflectance, a change of strength of reflected light which is accompanied with a change of surface shape corresponding to the presence or absence of pits (holes), or a change of polarized light surface by magnetooptic effect. These information recording carriers are advantageous because they can achieve high recording density and can perform non-contact recording-reproducing, and therefore can enjoy a long life.
Developments have been extensive in recent years in respect to cardlike information recording carriers (hereinafter referred to as optical cards) such that they now have good portability and a large capacity as compared with their size.
The optical recording medium is basically comprised of a substrate, a recording layer and a protective layer, and may further have various protective layers laminated for the purpose of improving thermal resistance, moisture resistance, light-resistance, abrasion resistance and so forth. Since scratches formed on the surface of the recording side face of the optical recording medium, or rubbish or dust stuck thereon, may cause a change of optical characteristics such as reflectance producing noise in reproducing signals, accordingly, it is necessary to form a hard protective layer coating having abrasion or scratch resistance; partcularly in the case of an optical card used in a form or environment such that the surface is liable to be scratched as compared with the optical disc.
For the purpose of increasing the recording density or enhancing the reliability of recording, an optical disc further generally has a complicated layer constitution such as the air-sandwich structure, and also has a substrate or protective layer of a sufficient thickness.
On the other hand, the optical card has a simple layer constitution in order to achieve a reduced thickness, and has a thinly formed substrate or protective layer.
However, because of the thinness of the upper layer of the recording layer, it is hard for an optical system for reproducing data to defocus the rubbish or dust stuck on the surface of the recording medium or the scratches made thereon, making it difficult to separate noise signals and recording signals. This is a disadvantage.